Fix editable install finder handling of nested packages

[abravalheri]

Summary of changes

• Only handle 1 level of nesting (necessary for the namespace packages to work properly)
• Rely on the importlib.machinery to find other levels of nested packages to find the correct files based on the parent path.

Closes #4019

Pull Request Checklist

• Changes have tests
• News fragment added in newsfragments/.
  (See documentation for details)

[abravalheri]

Docs likely to be breaking due to readthedocs/sphinx-notfound-page#219.
Temporary solution => disable extension.

[abravalheri]

There are still some errors with Sphinx. E.g. now it raises exceptions due to cyclic imports in setuptools code - which probably need further investigation:

CI logs

WARNING: autodoc: failed to import class 'Extension' from module 'setuptools'; the following exception was raised:
Traceback (most recent call last):
  File "/home/runner/work/setuptools/setuptools/.tox/docs/lib/python3.10/site-packages/sphinx/ext/autodoc/importer.py", line 64, in import_module
    return importlib.import_module(modname)
  File "/usr/lib/python3.10/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1050, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1027, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1006, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 6[88](https://github.com/pypa/setuptools/actions/runs/5900146250/job/16003850907?pr=4020#step:5:89), in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 883, in exec_module
  File "<frozen importlib._bootstrap>", line 241, in _call_with_frames_removed
  File "/home/runner/work/setuptools/setuptools/setuptools/__init__.py", line 17, in <module>
    from setuptools.dist import Distribution
  File "/home/runner/work/setuptools/setuptools/.tox/docs/lib/python3.10/site-packages/_virtualenv.py", line [90](https://github.com/pypa/setuptools/actions/runs/5900146250/job/16003850907?pr=4020#step:5:91), in exec_module
    old(module)
  File "/home/runner/work/setuptools/setuptools/setuptools/dist.py", line 37, in <module>
    from setuptools.config import setupcfg, pyprojecttoml
  File "/home/runner/work/setuptools/setuptools/setuptools/config/__init__.py", line 8, in <module>
    from . import setupcfg
  File "/home/runner/work/setuptools/setuptools/setuptools/config/setupcfg.py", line 38, in <module>
    from . import expand
  File "/home/runner/work/setuptools/setuptools/setuptools/config/expand.py", line 53, in <module>
    from setuptools.dist import Distribution  # noqa
ImportError: cannot import name 'Distribution' from partially initialized module 'setuptools.dist' (most likely due to a circular import) (/home/runner/work/setuptools/setuptools/setuptools/dist.py)

The above exception was the direct cause of the following exception:

reading sources... [ [92](https://github.com/pypa/setuptools/actions/runs/5900146250/job/16003850907?pr=4020#step:5:93)%] userguide/index
reading sources... [ [94](https://github.com/pypa/setuptools/actions/runs/5900146250/job/16003850907?pr=4020#step:5:95)%] userguide/miscellaneous
reading sources... [ [96](https://github.com/pypa/setuptools/actions/runs/5900146250/job/16003850907?pr=4020#step:5:97)%] userguide/package_discovery
reading sources... [ [98](https://github.com/pypa/setuptools/actions/runs/5900146250/job/16003850907?pr=4020#step:5:99)%] userguide/pyproject_config
reading sources... [[100](https://github.com/pypa/setuptools/actions/runs/5900146250/job/16003850907?pr=4020#step:5:101)%] userguide/quickstart
Traceback (most recent call last):
  File "/home/runner/work/setuptools/setuptools/.tox/docs/lib/python3.10/site-packages/sphinx/ext/autodoc/importer.py", line 88, in import_object
    module = import_module(modname, warningiserror=warningiserror)
  File "/home/runner/work/setuptools/setuptools/.tox/docs/lib/python3.10/site-packages/sphinx/ext/autodoc/importer.py", line 68, in import_module
    raise ImportError(exc, traceback.format_exc()) from exc
looking for now-outdated files... none found
ImportError: (ImportError("cannot import name 'Distribution' from partially initialized module 'setuptools.dist' (most likely due to a circular import) (/home/runner/work/setuptools/setuptools/setuptools/dist.py)"), 'Traceback (most recent call last):\n  File "/home/runner/work/setuptools/setuptools/.tox/docs/lib/python3.10/site-packages/sphinx/ext/autodoc/importer.py", line 64, in import_module\n    return importlib.import_module(modname)\n  File "/usr/lib/python3.10/importlib/__init__.py", line 126, in import_module\n    return _bootstrap._gcd_import(name[level:], package, level)\n  File "<frozen importlib._bootstrap>", line 1050, in _gcd_import\n  File "<frozen importlib._bootstrap>", line [102](https://github.com/pypa/setuptools/actions/runs/5900146250/job/16003850907?pr=4020#step:5:103)7, in _find_and_load\n  File "<frozen importlib._bootstrap>", line 1006, in _find_and_load_unlocked\n  File "<frozen importlib._bootstrap>", line 688, in _load_unlocked\n  File "<frozen importlib._bootstrap_external>", line 883, in exec_module\n  File "<frozen importlib._bootstrap>", line 241, in _call_with_frames_removed\n  File "/home/runner/work/setuptools/setuptools/setuptools/__init__.py", line 17, in <module>\n    from setuptools.dist import Distribution\n  File "/home/runner/work/setuptools/setuptools/.tox/docs/lib/python3.10/site-packages/_virtualenv.py", line 90, in exec_module\n    old(module)\n  File "/home/runner/work/setuptools/setuptools/setuptools/dist.py", line 37, in <module>\n    from setuptools.config import setupcfg, pyprojecttoml\n  File "/home/runner/work/setuptools/setuptools/setuptools/config/__init__.py", line 8, in <module>\n    from . import setupcfg\n  File "/home/runner/work/setuptools/setuptools/setuptools/config/setupcfg.py", line 38, in <module>\n    from . import expand\n  File "/home/runner/work/setuptools/setuptools/setuptools/config/expand.py", line 53, in <module>\n    from setuptools.dist import Distribution  # noqa\nImportError: cannot import name \'Distribution\' from partially initialized module \'setuptools.dist\' (most likely due to a circular import) (/home/runner/work/setuptools/setuptools/setuptools/dist.py)\n')

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/runner/work/setuptools/setuptools/.tox/docs/lib/python3.10/site-packages/sphinx/ext/autodoc/importer.py", line 64, in import_module
    return importlib.import_module(modname)
  File "/usr/lib/python3.10/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line [105](https://github.com/pypa/setuptools/actions/runs/5900146250/job/16003850907?pr=4020#step:5:106)0, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1027, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1006, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 688, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 883, in exec_module
  File "<frozen importlib._bootstrap>", line 241, in _call_with_frames_removed
  File "/home/runner/work/setuptools/setuptools/setuptools/__init__.py", line 35, in <module>
    __version__ = setuptools.version.__version__
AttributeError: partially initialized module 'setuptools' has no attribute 'version' (most likely due to a circular import)

WARNING: autodoc: failed to import class 'Command' from module 'setuptools'; the following exception was raised:
Traceback (most recent call last):
  File "/home/runner/work/setuptools/setuptools/.tox/docs/lib/python3.10/site-packages/sphinx/ext/autodoc/importer.py", line 64, in import_module
    return importlib.import_module(modname)
  File "/usr/lib/python3.10/importlib/__init__.py", line [126](https://github.com/pypa/setuptools/actions/runs/5900146250/job/16003850907?pr=4020#step:5:127), in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1050, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1027, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1006, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 688, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 883, in exec_module
  File "<frozen importlib._bootstrap>", line 241, in _call_with_frames_removed
  File "/home/runner/work/setuptools/setuptools/setuptools/__init__.py", line 35, in <module>
    __version__ = setuptools.version.__version__
AttributeError: partially initialized module 'setuptools' has no attribute 'version' (most likely due to a circular import)

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/runner/work/setuptools/setuptools/.tox/docs/lib/python3.10/site-packages/sphinx/ext/autodoc/importer.py", line 88, in import_object
    module = import_module(modname, warningiserror=warningiserror)
  File "/home/runner/work/setuptools/setuptools/.tox/docs/lib/python3.10/site-packages/sphinx/ext/autodoc/importer.py", line 68, in import_module
    raise ImportError(exc, traceback.format_exc()) from exc
ImportError: (AttributeError("partially initialized module 'setuptools' has no attribute 'version' (most likely due to a circular import)"), 'Traceback (most recent call last):\n  File "/home/runner/work/setuptools/setuptools/.tox/docs/lib/python3.10/site-packages/sphinx/ext/autodoc/importer.py", line 64, in import_module\n    return importlib.import_module(modname)\n  File "/usr/lib/python3.10/importlib/__init__.py", line 126, in import_module\n    return _bootstrap._gcd_import(name[level:], package, level)\n  File "<frozen importlib._bootstrap>", line 1050, in _gcd_import\n  File "<frozen importlib._bootstrap>", line 1027, in _find_and_load\n  File "<frozen importlib._bootstrap>", line 1006, in _find_and_load_unlocked\n  File "<frozen importlib._bootstrap>", line 688, in _load_unlocked\n  File "<frozen importlib._bootstrap_external>", line 883, in exec_module\n  File "<frozen importlib._bootstrap>", line 241, in _call_with_frames_removed\n  File "/home/runner/work/setuptools/setuptools/setuptools/__init__.py", line 35, in <module>\n    __version__ = setuptools.version.__version__\nAttributeError: partially initialized module \'setuptools\' has no attribute \'version\' (most likely due to a circular import)\n')

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/runner/work/setuptools/setuptools/.tox/docs/lib/python3.10/site-packages/sphinx/ext/autodoc/importer.py", line 64, in import_module
    return importlib.import_module(modname)
  File "/usr/lib/python3.10/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1050, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1027, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1006, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 688, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 883, in exec_module
  File "<frozen importlib._bootstrap>", line 241, in _call_with_frames_removed
  File "/home/runner/work/setuptools/setuptools/setuptools/__init__.py", line 35, in <module>
    __version__ = setuptools.version.__version__
AttributeError: partially initialized module 'setuptools' has no attribute 'version' (most likely due to a circular import)

/home/runner/work/setuptools/setuptools/docs/userguide/ext_modules.rst:5: WARNING: py:class reference target not found: setuptools.Extension

This is likely to be caused by sphinx-doc/sphinx#11608.

I think I will go ahead and pin sphinx<=7.1.2, because this PR is supposed to be a bugfix for a specific issue and there might be people depending on it. Investigation about cyclic dependencies will take more time and hold the process.